A pump of this type is especially known in document EP-A-0 453 357. This pump comprises a control rod traveling in a pump body. The piston in the form of an elastic moldable sleeve is mounted on the control rod. This piston slides in a tight or sealing manner against the inner wall of the pump body and likewise closes the intersecting ducts which lead to the inner duct of the control rod. The upper end of the sleeve which forms the piston is a catch support for the shoulder that forms the control rod. Thus, as soon as the pressure increases to a satisfactory level, the sleeve is formed by resting on the catch support for the control rod collar so that the pressurized fluid in the pump chamber flows through these intersecting ducts and the inner duct of the control rod. In the pump described in this document (EP-A-0 453 357), the piston/sleeve is therefore always subjected to stress which occurs at the top of the sleeve between the shoulder catch device of the control rod and the tight duct of the piston lip with the inner section of the pump body. Accordingly, the sleeve is likely to get out of shape with the effect of this permanent stress so that it no longer seals perfectly tightly the intersecting ducts of the control rod.
Another pump of the same type is known from document FR-1 544 683. The pump described in this document likewise includes a control rod on which is mounted a piston in the form of a sleeve. The piston comprises tightness lips with an impenetrable slide against the inner section of the pump body. The sleeve seals tightly the intersecting ducts which lead to the central duct of the control rod. In a manner similar to the document cited above, the top end of the sleeve supports the shoulder that forms the control rod. The sleeve comprises a section of less thickness which gives it a certain elastic deformability. This elastic deformability is for the purpose of clearing intersecting ducts when the inner pressure of the pump chamber reaches an adequate level. There again, the sleeve is permanently subjected to an axial stress which occurs between the shoulder support of the control rod and another lower support which likewise forms the control rod right below the intersecting ducts.
We are likewise familiar with other pumps of this type using a free floating sleeve piston mounted on the control rod. Certain ones use a precompression spring that require the piston to be in a position that is suitable for sealing the control rod intersecting ducts. We can specifically cite document FR-A-2 399 286. Other pumps using earlier techniques do not use any device that calls for the free floating piston so that there is no precompression.